docs: def historical(self, date, base='USD'):
          """Fetches historical exchange rate data from service
  
          :Example Data:
              {
                  disclaimer: "<Disclaimer data>",
                  license: "<License data>",
                  timestamp: 1358150409,
                  base: "USD",
                  rates: {
                      AED: 3.666311,
                      AFN: 51.2281,
                      ALL: 104.748751,
                      AMD: 406.919999,
                      ANG: 1.7831,
                      ...
                  }
              }
          """
          try:
              resp = self.client.get(self.ENDPOINT_HISTORICAL %
                                     date.strftime("%Y-%m-%d"),
                                     params={'base': base})
              resp.raise_for_status()
          except requests.exceptions.RequestException as e:
              raise OpenExchangeRatesClientException(e)
          return resp.json(parse_int=decimal.Decimal,
                           parse_float=decimal.Decimal)